Apparently, Black has problems with the pinned c-pawn, but he solves all of them with the next move.
18... a7b6  19. g2f3  19... a8a6 Removing the rook from the attack and defending the b6-pawn. Ding has accurately calculated the consequences of White's most direct move (20.Qb5), but there's another possibility. 20. b2b5 This lets any chance of an advantage slip. 20... a6a2 